Axial support system for a mr magnet

ABSTRACT

Four magnet cartridge axial supports are provided to prevent axial motion between the magnet cartridge and the surrounding vacuum vessel. Each axial support includes two high strength steel rods in parallel. The rods are brazed to a bar on one end which mounts to the magnet cartridge. The other end of the rods is brazed to a threaded stud which is secured by a bolt to a bracket secured to the vacuum vessel.

BACKGROUND OF THE INVENTION

The present invention relates to radial support systems for refrigeratedmagnetic resonance magnets.

The economical operation of a refrigerated superconducting magnetrequires that the heat load and hence the refrigeration load to themagnet windings be minimized. In cryostat systems which have a lightweight, thin walled thermal shield cooled by a cryocooler, it isadvantageous to support the thermal shield from the magnet cartridge.This allows the magnet cartridge and thermal shield to be assembledtogether prior to insertion in the vacuum vessel. The magnet cartridge,which is the primary mass within the enclosure, is independentlysupported by the vacuum vessel.

In order to minimize the conduction heat leak from the vacuum vessel andthermal shield to the magnet cartridge, it is necessary to use supportsmade with low thermal conductivity materials, minimized cross sectionalarea, and maximized length. The supports must be designed to minimizethe deformation of the magnet cartridge and the thermal shield due tothe reaction forces at the attachment points of the supports. Thesupports must permit the axial and radial positioning of the magnetcartridge and the thermal shield within the vacuum vessel. Theadjustment mechanisms must be compatible with the overall cryostatassembly procedure.

During the cool down of the magnet from room temperature to operatingtemperature, there is normally a finite differential thermal contractionbetween the magnet cartridge, the thermal shield, and the outer vacuumvessel. The supports must accommodate this differential thermalcontraction and maintain adequate support at operating temperatures.

The supports must be capable of withstanding the forces due to thedynamic shock loading of the system during shipping and handling. Shockloads during shipping have been measured at approximately 2 g vertical,and 1 g horizontal.

During the ramp up of a magnet during which the current in the windingsis increased to their operating level, the magnet cartridge typicallyexpands radially due to magnetic forces. There must be no frictionalheating between the magnet cartridge and the supports during ramp up, asthis could induce a local temperature rise in the magnet and lead to amagnet quench (transition from superconducting to normal resistance inthe superconductive windings).

It is an object of the present invention to provide an axial supportsystem to minimize heat input to the magnet cartridge while maintainingadequate mechanical strength and adjustment capability.

SUMMARY OF THE INVENTION

A superconductive MR magnet is provided including a generallycylindrical vacuum vessel defining an axially extending bore. A magnetcartridge having a cylindrical shape is situated in the vacuum vesselconcentric with and spaced away from the bore of the magnet. An axialmagnet cartridge support comprising a first tension rod is affixed toone end of the magnet cartridge and extends axially along the magnetcartridge, radially spaced away therefrom, and is affixed to the vacuumvessel by tension adjusting means. A second tension rod affixed to theopposite end of the magnet cartridge extends axially along the magnetcartridge radially spaced away therefrom, and is affixed to the vacuumvessel by tension adjusting means. Each of the rods limit axial motionin one direction.

BRIEF DESCRIPTION OF THE DRAWING

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the followingdescription taken in conjunction with accompanying figures in which:

FIG. 1 is an isometric view with a cutaway of the vacuum vessel showingthe position of the magnet cartridge radial struts and the shield axialsupports in accordance with the present invention;

FIG. 2 is a partial sectional end view taken along the lines 2--2 ofFIG. 1 showing the lateral magnet cartridge radial struts and magnetcartridge axial struts on the cryocooler side of the vacuum vessel;

FIG. 3 is a partial sectional end view taken along the lines 3--3 ofFIG. 1 showing the vertical magnet cartridge radial struts and magnetcartridge axial struts;

FIG. 4 is a partial side view taken along the lines 4--4 of FIG. 1 ofone of the magnet cartridge radial struts of a radial strut pair;

FIG. 5 is a partial side view taken along the lines 5--5 of FIG. 1 ofthe other magnet cartridge radial strut of the radial strut pair; and

FIG. 6 is a partial side view taken along the lines 6--6 of the axialthermal shield supports.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawing wherein like numerals indicate likeelements throughout and particularly FIGS. 1, 2 and 3 thereof, arefrigerated superconductive magnetic resonance (MR) magnet 11 is shown.The magnet includes a cylindrical vacuum vessel 13 having an axiallyextending bore 15. Located inside the vacuum vessel is a cylindricalmagnet cartridge 17 surrounded by a thermal radiation shield 21. Themagnet cartridge contains a plurality of windings symmetrically disposedabout the axial centerline of the cartridge. In the present embodimentthree pairs of superconductive Nb₃ Sn windings are wound on a fiberglassreinforced form which has been machined by provide circumferential slotsfor winding the superconductive coils and axial slots for electrical busbars connecting the coils together. A winding of this type is shown andclaimed in copending application entitled "Superconductive QuenchProtected Magnet Coil", Serial No. 07/412,254 a continuation of07/215,479, now abandoned and hereby incorporated by reference. Astainless steel end ring 23 is affixed to either end of the cartridge bythreaded bolts and epoxy resin bonding.

The vacuum vessel has a cylindrical extension 25 which protrudesradially outwardly from the vacuum vessel in the horizontal direction.The central axis of the extension lies on a radial line extendingthrough the axial midplane of the vacuum vessel. The cylindricalextension has an annular shaped cover 27. A two stage cryocooler 31 ismounted to the cover with the cold end of the cryocooler extendinginside the vacuum vessel. The cryocooler 31 is mounted using aninterface of the type shown and claimed in copending application Ser.No. 07/348,322, now U.S. Pat. No. 4,093,318 entitled "Cryocooler ColdHead Interface Receptacle". The cryocooler cold end 32, visible in FIG.2, typically transmits a force of approximately 1000 pounds on thecenter portion of the magnet cartridge when the vacuum vessel isevacuated. The vacuum vessel and the cylindrical extension can befabricated from carbon steel with the bore sleeve fabricated fromstainless steel, for example.

There are six magnet cartridge radial struts supporting the magnetcartridge from the vacuum vessel. Two of the struts are arranged in thevertical direction and are affixed at either axial end of the magnetcartridge in the horizontal plane on the side of the cartridge oppositethe cryocooler. Vertical strut 33 is shown in FIG. 3. Four of the magnetcartridge struts extend laterally. Two lateral struts 35 and 37 areshown in FIG. 2 and are affixed to one end of the magnet cartridge aboveand below the horizontal plane forming an acute angle when measured fromthe horizontal plane. The other end of both struts 35 and 37 are affixedto the vacuum vessel with strut 35 extending upwards at an angle andstrut 37 extending downwards at an angle toward the vacuum vessel. Theother two lateral struts are affixed to the other axial end of themagnet cartridge above and below the horizontal plane symmetricallylocated about the horizontal midplane of the vacuum vessel on thecryocooler side of the magnet.

Each of the magnet cartridge radial struts comprise a thin walled, G-10fiberglass epoxy cylinder 39 which has internal threads machined ineither end. The central portion of the fiberglass cylinder is machinedto reduce its outside diameter or wall thickness and therefore its heatconductance. The transition between the narrow central portion and theends has a 3/8 inch radius stress riser. The threads in either end stopone quarter of an inch before the reduced diameter portion of thecylinder begins. Threaded into either end of the fiberglass epoxycylinder is a multiaxis joint 41 such as a ball joint of the typeavailable from Aurora Bearing Corp., Aurora, Ill. The struts are securedto the magnet cartridge by axially extending shoulder bolts 43 passingthrough the multiaxis joint threadingly engaging ring 23. The other endof each of the radial struts is attached to a clevis 45. The verticalstruts each pass through openings in the thermal shield and the vacuumvessel, and are surrounded by vertical cylindrical extensions 47 and 49and the vacuum vessel. The lateral struts each pass through openings inthe thermal shield and vacuum vessel and are surrounded by cylindricalextensions 51, 52, 53 and 54. Part of the clevis 45 is used to close offthe end of the cylindrical extension. The lateral extending radialstruts pass through openings in the vacuum vessel and throughcylindrical extensions closed off by a portion of the clevis which isattached to the ends of the strut.

The vertical and lateral struts are thermally stationed to the thermalshield 21 at an intermediate location in order to intercept some of theconduction heat from the vacuum vessel which is at ambient temperatureand carry it to the shield which is cooled to approximately 40° K. bythe cryocooler 31. The thermal shield can be fabricated from a heatconductive material such as aluminum. Copper braid 55 is epoxy bonded tothe strut and soldered or bolted (with an appropriate interfacematerial) to the shield to conduct the heat from the strut to theshield.

In operation, the two vertical support struts 33 support half of theweight of the magnet cartridge 17. The magnet cartridge in a 0.5 Tmagnet typically weighs approximately 1,000 lbs. The two pairs oflateral struts 35 and 37 react against the contact force of thecryocooler cold head and support half of the weight of the cartridge.The multiaxis joints 41 of the vertical supports pivot to allow themagnet cartridge axial centerline to move due to the thermal shrinkagetowards the cryocooler 31 during cool down. The lateral struts do notpermit unrestricted motion in the radial direction in the horizontalplane. The initial position of the magnet cartridge is offset so thatwhen shrinkage of the magnet cartridge occurs, the vertical struts willbe in the vertical position. The magnet cartridge shrinks radiallyinward when cooled. Since the lateral struts prevent movement of oneside of the magnet cartridge, the center and opposite side of the magnetcartridge move towards the cryocooler. All the radial struts areattached to the magnet cartridge near the horizontal center plane inorder to minimize the sag of the cartridge due to its own weight. Duringassembly, the ends of the vacuum vessel are not yet in place. The magnetcartridge 17 and thermal shield 21 are positioned in the vacuum vessel13. The clevises 45 are affixed to the warm end of the struts, and arepositioned through the cylindrical extensions into the vacuum vessel.The thermal braid heat stations 55 are attached to the thermal shieldand the struts are secured to the magnet cartridge using shoulder bolts43. The cryocooler 31 is installed, or a radially inward forceequivalent to the crycooler is imposed on the magnet cartridge when theradial struts are adjusted. Alternatively, the cryocooler is notinstalled, but the deformation of the magnet cartridge is anticipatedduring radial strut adjustment. Once installed the radial struts carrythe radial load while being adjusted. The clevises are rotated toachieve radial adjustment of the magnet cartridge position. Rotating theclevis adjusts the overall length between the threaded multiaxissupports of each strut. When the radial adjustment is achieved, eachclevis mount is welded to each end of the cylindrical extension to forma vacuum seal.

The angle chosen for the lateral struts depends on the load imposed, aswell as the length of the strut desired to minimize the conduction heatload to the magnet cartridge. A longer strut provides less heatconduction. Flexibility in angle selection is available: increasingangles measured relative to the horizontal plane increase the length ofthe struts, the load on the struts, and the size of the opening in thethermal shield.

Referring now to FIGS. 4 and 5, two of the four magnet cartridge axialsupports 61 and 63 are shown. Each support comprises two high strengthsteel rods 65 in parallel. Both rods can be seen in FIGS. 2 and 3. If asingle wire having sufficient strength is available it can alternativelybe used. The rods are brazed on one end into a bar 67 which mounts tothe magnet cartridge end ring 23 by means of a threaded stud with ashoulder 71. The stud is held in place with a roll pin 73. The bar andstud attachment to the cartridge are designed to have a minimumthickness radially in order to fit within the annular gap between thecartridge and the thermal shield.

The other end of the steel rods are brazed into a threaded stud 75. Thisstud is bolted by nut 77 to a bracket 81 which is mounted to the vacuumvessel. The bracket allows the radial position of the stud to beadjusted, if necessary. The threaded stud is sufficiently long to permitthe axial positioning of the cartridge within the vacuum vessel. Therods are prevented from thermally shorting to either the cartridge orthe shield along the midlength by five equally spaced thermal standoffs83 made of 0.010 inch G-10. The standoffs insure that the rods are notdirectly contacting either surface, and conduct only minute amounts ofheat from the shield or to the cartridge. Wires provide an inexpensiveaxial support. Having separate radial and axial magnetic cartridgesupports simplify assembly and adjustment. Long thin wires minimize theheat load and do not require thermal stationing to the thermal shield.

The magnet cartridge axial supports are mounted in pairs with a pairdiametrically opposite the first pair shown in FIGS. 5 and 6. Each axialsupport prevents axial motion in one axial direction. The pair preventsaxial motion in either direction.

Rotation about the vertical axis of the magnet cartridge is prevented bythe radial struts and axial cables. Rotation about the radial directionlying in the horizontal plane of the magnet cartridge is prevented bythe lateral struts. Rotation about the axially extending axis isprevented by the vertical and lateral radial struts.

Thermal shield radial bumpers 85 shown in FIG. 2 are described incopending application Serial No. 07/215,111, now U.S. Pat. No. 4,935,714entitled "Low Thermal Conductance Support for a Radiation Shield in a MRMagnet", are used to support the thermal shield from the magnetcartridge. U.S. Pat. No. 4,935,714, "Low Thermal Conductance Support fora Radiation Shield in a MR Magnet" is hereby incorporated by reference.

Two of the four thermal shield axial supports 87 are shown in FIG. 6. Asingle strand of wire 91 is brazed into threaded studs 93 and 95 on eachend. Stud 95 is threaded into a tapped hole in ring 23 on the end of themagnet cartridge. Flats 97 are machined on the stud prior to brazing inorder to facilitate the insertion and removal of the stud. Stud 93extends through a clearance hole in the end flange of the thermal shield21. Belleville spring washers 101 are compressed against the shield by anut 103. Stud 93 has machined flats 104 to facilitate the tightening ofthe nut without twisting the wire. A collar 105 between the stud 93 andthe stack of spring washers 101 aligns the washers and prevents contactwith the stud threads. Stud 93 is of sufficient length to permitadjustability of the axial position of the shield and to accommodatedimensional tolerance stackups. Two additional axial supports (notshown) are used on either end of the magnet cartridge, mounted 180 aparton the end flanges.

Prior to cool down of the magnet cartridge 17 the Belleville washers 101are compressed. As the shield 21 and magnet cartridge 17 cool down, theshield contracts axially with respect to the magnet cartridge, thusrelaxing the washer stacked compression. The wire tension relaxes fromthe maximum pretension to a nominal tension at operating temperature.Wires provide an inexpensive axial support. Having separate radial andaxial shield supports simplify initial adjustments.

The foregoing has described a radial support system to minimize heatinput to the magnet cartridge while maintaining adequate mechanicalstrength and adjustment capability.

While the invention has been particularly shown and described withreference to an embodiment thereof, it will be understood by thoseskilled in the art that various changes in form and detail may be madewithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A superconductive MR magnet comprising:agenerally cylindrical vacuum vessel defining an axially extending bore;a magnet cartridge having a cylindrical shape and situated in the vacuumvessel concentric with and spaced away from the bore of the magnet; andan axial magnet cartridge support comprising a first tension rod affixedto one end of the magnet cartridge and extending axially along themagnet cartridge radially spaced away therefrom, and affixed to thevacuum vessel by tension adjusting means, and a second tension rodaffixed to the opposite end of the magnet cartridge and extendingaxially along the magnet cartridge radially spaced away therefrom, andaffixed to the vacuum vessel by tension adjusting means, each of saidrods limiting axial motion in one direction.
 2. The refrigeratedsuperconductive magnet of claim 1 wherein said tension adjusting meanscomprises a threaded rod secured to one end of each of said rods, abracket defining an aperture mounted to said vacuum vessel, saidthreaded rod passing through the aperture, and a nut threadinglyengaging said rod for adjusting the rod tension.
 3. The refrigeratedsuperconductive magnet of claim 2 further comprising spacer means of lowthermal conductivity material situated between said wire and said magnetcartridge and said vacuum vessel.